EP1074391B1

EP1074391B1 - Thermal print head and method of manufacture thereof

Info

Publication number: EP1074391B1
Application number: EP00902064A
Authority: EP
Inventors: Takumi Rohm Co. Ltd YAMADE; Hiroaki Rohm Co. Ltd HAYASHI; Teruhisa Rohm Co. Ltd SAKO
Original assignee: Rohm Co Ltd
Current assignee: Rohm Co Ltd
Priority date: 1999-02-18
Filing date: 2000-02-02
Publication date: 2007-04-04
Anticipated expiration: 2020-02-02
Also published as: JP3990112B2; DE60034186D1; US6331868B1; KR100359635B1; CN1294555A; WO2000048839A1; DE60034186T2; TW506915B; EP1074391A4; CN1108239C; KR20010042785A; EP1074391A1

Abstract

A thermal print head (A) comprises an insulating substrate (10) with upper and side surfaces (10a, 10b), and a glazed layer (11) formed to the substrate (10) for keeping heat. A heating resistor (13) is formed on the glazed layer (11). The thermal print head (A) futher includes a common electrode (12) and a plurality of individual electrodes. The common electrode (12) has a plurality of teeth (12a) for connection with the heating resistor (13) and a tie (12b) that connects the teeth (12a). An auxiliary electrode layer (14) is formed on the tie (12b). The heat resistor (13) and the auxiliary electrode layer (14) are covered with an overcoating layer (16), which is covered with a protective layer (17). The tie (12b) of the common electrode (12) directly touches both the glazed laer (11) and the upper surface (10a) of the substrate.

Description

The present invention relates to a thermal printhead, or specifically a thick-film thermal printhead. It also relates to a method of making such a thermal printhead.

BACKGROUND ART

As is well known, a thick-film thermal printhead has a heating resistor and an electrode pattern (including a common electrode and individual electrodes) formed by printing and baking a conductive paste.
US 5,485,192 discloses a thermal printhead comprising an insulating head substrate, a conductor pattern formed on the substrate, a row of heating dots formed on the substrate in electrical conduction with the conductor pattern and a protective coating comprising a smaller thinner portion at the row of heating dots and a larger thickness portion in contact with a resin body protecting an array of drive ICs.
JP-A-1,255,565 discloses a thermal printhead with a substrate having a tapered end, an insulating layer is coated onto the substrate, but stops short of the termination of the taper. The insulating layer then has a resistance film and a pair of electrode films shaped onto the outer surface and a protective layer covers these layers and reaches to the end of the taper.
Fig. 11 of the accompanying drawings is a sectional view showing a further example of a prior art thermal printhead. The illustrated thermal printhead B includes a substrate 100 which is provided, entirely on the upper surface thereof, with a glaze layer 110 for heat retention. A common electrode 120 and a plurality of individual electrodes (not shown) are formed on the glaze layer 110. The thermal printhead B further includes a heating resistor 130 electrically connected to the common electrode 120 and the individual electrodes.
An common electrode auxiliary layer 140 is formed on the common electrode 120 at a portion spaced from the heating resistor 130. The common electrode auxiliary layer 140 is provided for preventing a voltage drop in the common electrode 120.
The thermal printhead B has an overcoat layer 150 for covering the common electrode 120, non-illustrated individual electrodes, the heating resistor 130 and the common electrode auxiliary layer 140. Further, a protective layer 160 which is thinner than the overcoat layer 150 is formed on the overcoat layer 150. The protective layer 160 is formed of a material which is less susceptible to wear and scratches than the material for the overcoat layer 150. With such a structure, the common electrode 120 and other parts are prevented from directly contacting a recording paper S. As shown in Fig. 11, the protective layer 160 is formed not only on the upper surface of the overcoat layer 150 but also continuously on a side surface 100s of the substrate 100.
As shown in Fig. 11, a platen roller C is provided on the thermal printhead B so as to contact the protective layer 160. The platen roller C rotates in the direction of the arrow D1 to transfer the recording paper S in the direction of the arrow D2 in close contact with the protective layer 160. At this time, the recording paper S transferred outwardly by the platen roller C warps downwardly by its own weight. The substrate 100 and the glaze layer 110 are chamfered in such a manner as to correspond to such a warp. As a result, the substrate 100 is formed with a first bevel portion 100a, whereas the glaze layer 110 is formed with a second bevel portion 110a. Accordingly, it is possible to prevent the recording paper S from being caught at a corner of the substrate 100 (or of the glaze layer 110), and therefore, it is possible to transfer the recording paper S smoothly outwardly by the platen roller C.
While having the advantages described above, the prior art thermal printhead B has the following problems.
First, due to the difference in thermal expansion coefficient between the glaze layer 110 and the protective layer 160, the protective layer 160 in the form of a thin film may break or may be released from the glaze layer 110. Specifically, the protective layer 160 directly covers the glaze layer 110 at a portion of the upper surface and continuously at the inclined portion 110a. When the glaze layer 110 and the protective layer 160 are heated, they thermally expand to different degrees with each other. As a result, stress is concentrated on the ridge 160a of the protective layer 160, resulting in the breakage of the protective layer 160.
Secondly, with the structure of the prior art thermal printhead B, it is impossible to sufficiently urge the recording paper S toward the heating resistor 130, which may cause improper printing. As shown in Fig. 11, the protective layer 160 has a first convex portion 160b (a portion above the heating resistor 130) and a second convex portion 160c (a portion above the common electrode auxiliary layer 140), with both of which convex portions the platen roller C engages. However, because of the existence of the common electrode auxiliary layer 140, the second convex portion 160c is located considerably higher than the first convex portion 160b (See the sign "t" in the drawing). With such a structure, the pressing force by the platen roller C is mostly exerted on the second convex portion 160c, so that the recording paper S is not sufficiently pressed against the first convex portion 160b. As a result, heat from the heating resistor 130 is not sufficiently transmitted to the recording paper S, which may cause printing failure such as unclear printing results.

DISCLOSURE OF THE INVENTION

The present invention, which is conceived under the circumstances described above, aims to provide a thermal printhead which is capable of preventing a thin film protective layer on a bevel surface of a substrate from being peeled off or broken and which is capable of performing printing at suitable density.
Another object of the present invention is to provide a method of making such a thermal printhead.
According to a first aspect of the present invention there is provided a thermal printhead as claimed in claim 1, and the claims dependant thereon. The thermal printhead comprises: an insulating substrate having an upper surface and a side surface; a glaze layer for heat retention formed on the upper surface of the substrate; a heating resistor formed on the glaze layer; a common electrode having a plurality of teeth in connected to the heating resistor, and a connecting portion connecting the teeth with each other; a plurality of individual electrodes connected to the heating resistor; an electrode auxiliary layer formed on the connecting portion of the common electrode; an overcoat layer for covering the heating resistor and the electrode auxiliary layer; and a protective layer for covering the overcoat layer; wherein the connecting portion of the common electrode includes a first region contacting the glaze layer and a second region contacting the upper surface of the substrate.
Preferably, the electrode auxiliary layer contacts both the first region and the second region of the connecting portion.
Preferably, the electrode auxiliary layer includes a thinner portion contacting the first region of the connecting portion and a thicker portion contacting the second region of the connecting portion.
According to a preferred embodiment of the present invention, the protective layer includes a first protrusion positionally corresponding to the heating resistor and a second protrusion positionally corresponding to the thinner portion of the electrode auxiliary layer. The first and the second protrusions are substantially equal in height.
Preferably, the glaze layer includes an uneven portion contacting the first region of the connecting portion, and the uneven portion is tapered toward the side surface of the substrate.
According to a preferred embodiment of the present invention, the substrate has a bevel surface extending between the upper surface and the side surface of the substrate.
Preferably, the glaze layer is spaced from the bevel surface.
Preferably, the bevel surface is covered with a protective layer.
Preferably, the bevel surface is roughened.
According to a second aspect of the invention there is provided a method of making a thermal printhead as claimed in claim 10 and the claims dependant thereon.
The method of making a thermal printhead relates to a thermal printhead which comprises an insulating substrate having an upper surface and a second surface adjoining the upper surface; a heat retaining glaze layer formed on the upper surface of the substrate; a heating resistor formed on the glaze layer; an electrode pattern (comprising a plurality of teeth and a connecting portion) connected to the heating resistor; an electrode auxiliary layer formed on the electrode pattern; an overcoat layer for covering the heating resistor and the electrode auxiliary layer; and a protective layer formed on the overcoat layer. The method comprises the steps of: forming the glaze layer to be spaced from the second surface of a substrate; forming the electrode pattern to have a first region contacting the glaze layer and a second region contacting the upper surface of the substrate; forming the electrode auxiliary layer so as to contact both the first region and the second region of the electrode pattern; forming the overcoat layer to be spaced from the second surface of the substrate; and forming the protective layer to cover the overcoat layer and the second surface of the substrate.
Preferably, the glaze layer is formed to have an uneven portion tapered toward the second surface of the substrate, and the first region of the electrode pattern is formed to contact the uneven portion.
According to a preferred embodiment of the present invention, the step of forming the electrode auxiliary layer includes applying a fluid conductive paste onto both the first and the second regions of the electrode pattern.
Preferably, the conductive paste is allowed to flow from the first region to the second region.
Preferably, the second surface of the substrate is a bevel surface expending between the upper surface and a side surface of the substrate.
Preferably, the method further comprises the step of working the substrate to provide a bevel surface.
When the substrate is obtained by dividing an insulating support the method comprises intermediate steps between the steps of forming the auxiliary layer and forming the overcoat layer; the intermediate steps are; cutting the support at a position spaced from the electrode pattern and the electrode auxiliary layer and chamfering the support to have a bevel surface which is spaced from the electrode pattern and the electrode auxiliary layer.
According to a preferred embodiment of the present invention, the method further comprises the step of applying laser to the support from below to form a cutting guide groove for cutting the support.
Preferably, the protective film is formed of a material containing sialon.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a plan view showing a principal portion of a thermal printhead embodying the present invention.
Fig. 2 is a sectional view taken along lines I-I in Fig. 1.
Figs. 3~6 are perspective views showing an example of method of making a thermal printhead in accordance with the present invention.
Figs. 7~10 are perspective views showing another example of method of making the thermal printhead in accordance with the present invention.
Fig. 11 is a sectional view showing a prior art thermal printhead.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described below in detail with reference to Figs. 1 through 10.
As shown in Figs. 1 and 2, a thermal printhead A in accordance with the present invention includes an insulating substrate 10, a glaze layer 11 for heat retention, a common electrode 12, a heating resistor 13, a common electrode auxiliary layer 14, a plurality of individual electrodes 15, an overcoat layer 16, and a protective layer 17. The thermal printhead A, which is held in close contact with a platen roller C (Fig. 2), is incorporated in a printing apparatus.
The substrate 10 may be formed of a ceramic material for example. Though not illustrated in Figs. 1 and 2, the substrate has an elongated, substantially rectangular configuration. The heating resistor 13 extends longitudinally of the substrate 10. As shown in Fig. 2, the substrate 10 has an upper surface 10a and a side surface 10b. The corner defined by the upper surface 10a and the side surface 10b of the substrate 10 is chamfered to provide a bevel surface 10c for transition from the upper surface 10a to the side surface 10b of the substrate 10. Preferably, the bevel surface 10c is roughened.
As shown in Fig. 2, the glaze layer 11 is formed directly on the upper surface 10a of the substrate 10. Further, as shown in Fig. 1, the glaze layer 11 has an edge 11a extending in parallel to the heating resistor 13. The edge 11a is spaced from the bevel surface 10c of the substrate 10. As is clear from Fig. 2, the glaze layer 11 comprises an even portion 11b having a constant thickness and an uneven portion 11c which varies in thickness. The uneven portion 11c tapers toward the edge 11a.
The common electrode 12 (to be exact, a portion of the common electrode 12) and the individual electrodes 15 are formed on the glaze layer 11. As shown in Fig. 1, the common electrode 12 has a plurality of teeth 12a, and a connecting portion 12b connecting these teeth together. The teeth 12a of the common electrode 12 and the individual electrodes 15 are alternately disposed. The teeth 12a and the individual electrodes 15 extend transversely of the heating resistor 13. The heating resistor 13 extends over each tooth 12a and one end of each individual electrode 15 in electrical connection thereto. Though not illustrated, the other end of each individual electrode 15 is electrically connected to a corresponding output pad of a drive IC.
As can be seen from Figs. 1 and 2, the connecting portion 12b of the common electrode 12 is so formed as to directly contact both the glaze layer 11 and the substrate 10. Specifically, the connecting portion 12b includes a first region directly contacting the glaze layer 11, and a second region directly contacting the upper surface of the substrate 10. The connecting portion 12b does not reach the bevel surface 10c of the substrate 10. Specifically, the connecting portion 12b extends on the uneven portion 11c of the glaze layer 11 toward the bevel surface 10c of the substrate 10, but terminates between the edge 11a of the glaze layer 11 and the bevel surface 10c.
The common electrode auxiliary layer 14 is elongated similarly to the heating resistor 13 and attached to the connecting portion 12b of the common electrode 12. The common electrode auxiliary layer 14 is provided for reducing a voltage drop in the common electrode 12. Similarly to the connecting portion 12b of the common electrode 12, the common electrode auxiliary layer 14 is inclined and extends across the edge 11a of the glaze layer 11, as shown in Fig. 2. The common electrode auxiliary layer 14 is not constant in thickness, i.e., it has a smaller thickness on the uneven portion 11c of the glaze layer 11 than on the other portions.
The overcoat layer 16 is formed to cover the common electrode 12, the heating resistor 13, the common electrode auxiliary layer 14 and the individual electrodes 15. The overcoat layer 16 may be formed of a material mainly composed of glass by a known thick film technique. The protective layer 17 is formed to cover the overcoat layer 16. The protective layer 17 may be formed by a known thin film technique. As shown in Fig. 2, the overcoat layer 16 does not extend up to the bevel surface 10c of the substrate 10 beyond the connecting portion 12b of the common electrode 12. On the other hand, the protective layer 17 extends not only onto the upper surface 10a but also onto the bevel surface 10c and the side surface 10b of the substrate 10. As described above, the bevel surface 10c is preferably roughened so that the protective layer 17 is firmly attached on the bevel surface 10c.
In the thermal printhead A according to the present invention, the platen roller C comes into contact with two convex portions of the protective layer 17, i.e., a first convex portion 17a (positionally corresponding to the heating resistor 13) and a second convex portion 17b (positionally corresponding to the thinner portion of the common electrode auxiliary layer 14), as also is the case in the prior art thermal printhead B (Fig. 11). However, the second convex portion 17b of the thermal printhead A does not bulge as much as conventionally is, because the common electrode auxiliary layer 14 laid beneath the second convex portion has a (partially) smaller thickness. As a result, the difference in height (T) between the first convex portion 17a and the second convex portion 17b is minimal (substantially 0).
With such a structure, it is possible to exert a pressing force of the platen roller C effectively onto the first convex portion 17a. Accordingly, the recording paper S is pressed against the first convex portion 17a with a sufficient urging force. As a result, heat generated at the heating resistor 13 is effectively transmitted to the recording paper S, thereby providing good printing results.
Next, a method of making the thermal printhead A in accordance with the present invention will be described with reference to Figs. 3 through 6. As will be understood from the description given below, it possible, according to this method, to obtain a plurality of thermal printheads A from a single mother board.
First, as shown in Fig. 3, a mother board 20 is prepared on which a groove 21 having a triangular cross section is formed. In this way, bevel surfaces 21a are provided on the mother board 20. As will be easily understood, each of the bevel surfaces 21a becomes a bevel surface 10c of each individual substrate 10 (obtained by dividing the mother board 20). The bevel surface 21a is preferably roughened.
Then, as shown in Fig. 4, a glaze layer 11 is formed so as not to reach the bevel surface 21a. The glaze layer 11 has an edge 11a spaced from the bevel surface 21a by a predetermined distance.
Subsequently, as shown in Fig. 5, a common electrode 12 is formed using photolithography which includes an etching step for example. At this time, though not illustrated in Fig. 5, a plurality of individual electrodes 15 are also formed. The common electrode 12 is formed to have teeth which are entirely located on the glaze layer 11, and a connecting portion 12b which is partially located on the glaze layer with the remaining portions located on the upper surface of the mother board 20 (substrate 10). The remaining portions of the connecting portion 12b do not reach the bevel surface 21a.
Then, as shown in Fig. 6, an common electrode auxiliary layer 14 is formed on the connecting portion 12b of the common electrode 12. The common electrode auxiliary layer 14 also extends across the edge 11a of the glaze layer 11. The common electrode auxiliary layer 14 may be formed by applying a conductive paste containing gold, palladium and/or silver and solidifying the paste.
The conductive paste has fluidity before solidification. Accordingly, when the conductive paste is applied onto the common electrode 12 at the connecting portion 12b which is smoothly inclined toward the bevel surface 21a, the conductive paste tends to move (flow) toward the bevel surface 21a, so that the amount of the conductive paste retained on the uneven portion 11c (See Fig. 2) of the glaze layer 11 becomes less than the amount of the conductive paste built up between the edge 11a of the glaze layer 11 and the bevel surface 21a. Thus, upon solidification, the conductive paste (i.e., the common electrode auxiliary layer 14) provides a smaller thickness on the uneven portion 11c of the glaze layer 11 and a larger thickness at the remaining portions.
Then, a heating resistor 13 is formed to extend transversely to the teeth 12a of the common electrode 12 and the individual electrodes 15 (See Fig. 1). The heating resistor 13 may be formed by applying a paste material having a predetermined resistivity and then solidifying the paste material.
Subsequently, an overcoat layer 16 is formed as a thick film to cover the heating resistor 13 and the common electrode auxiliary layer 14 without reaching the bevel surface 21a of the mother board 20 (See Fig. 2).
After the overcoat layer 16 is formed, the mother board 20 is divided into a plurality of individual substrates 10. Then, a protective layer 17 is formed as a thin film on the substrate 10 by spattering for example. As shown in Fig. 2. the protective layer 17 covers not only the overcoat layer 16 but also the bevel surface 10c and the side surface 10b of the substrate 10.
As will be appreciated, in an alternative both the overcoat layer 16 and the protective layer 17 may be formed on the individual substrate 10 after dividing the mother board 20. In a further alternative, the overcoat layer 16 may be formed immediately short of the bevel surface 10c.
Unlike the prior art thermal printhead B (Fig. 11), in the thermal printhead A obtained through the above process, the protective layer 17 and the glaze layer 11, which are different from each other in thermal expansion coefficient, are not held in contact with each other. Instead, the protective layer 17 is formed directly on the substrate 10 at or near the bevel surface 10c. With such a structure, it is possible to reliably prevent the protective cover 17 from being broken (or peeled off) at the bevel surface 10c of the substrate 10.
Now, reference is made to Figs. 7 through 10. Each of these figures is a perspective view showing another method of forming the thermal printhead according to the present invention.
First, in this method, an insulating mother board 20' is prepared on which a glaze layer 11' is formed, as shown in Fig. 7. Similarly to the method described above, the glaze layer 11' is formed to have a linearly extending edge 11a'.
Subsequently, as shown in Fig. 8, a common electrode 12' is formed using photolithography which includes an etching step for example. At this time, though not illustrated in the figure, a plurality of individual electrodes are also formed. The common electrode 12' is formed to have teeth which are entirely located on the glaze layer 11', and a connecting portion 12b' which is partially located on the glaze layer 11' with the remaining portions located on the upper surface of the mother board 20'. After the common electrode 12' and the individual electrodes are formed, a heating resistor (not shown) is formed to extend transversely to the teeth of the common electrode 12' and the individual electrodes. However, the heating resistor need not necessarily be formed at this stage. Alternatively, the heating resistor may be formed at the same time as the formation of a common electrode auxiliary layer which will be described below, or after the formation of a common electrode auxiliary layer.
Then, as shown in Fig. 9, an common electrode auxiliary layer 14' is formed on the connecting portion 12b' of the common electrode 12'. Similarly to the connecting portion 12b' of the common electrode 12', the common electrode auxiliary layer 14' also extends across the edge 11a' of the glaze layer 11'.
After the common electrode auxiliary layer 14' is formed, the mother board 20' is divided along a cutting line CL shown in Fig. 9. In this way, a plurality of individual substrates 10' are provided. Though not illustrated, each of the substrates 10' is similarly formed with the glaze layer and the electrode pattern.
The division of the mother board 20' may be performed in the following manner for example. First, as indicated by an arrow in Fig. 9, laser is applied to the mother board 20' from below to form a cutting guide groove on the lower surface of the mother board 20'. Then the mother board 20' is divided by a suitable cutting means along the cutting guide groove. Alternatively, after a cutting guide groove is formed, a bending force may be applied on the mother board 20' to divide the mother board. In such a case, no cutting means is necessary.
Then, the upper corner portion of the substrate 10' is beveled. As a result, a bevel surface 10c' is provided which extends between the upper surface and side surface 10b' of the substrate 10'. The bevel surface 10c' is so formed as to be spaced from the connecting portion 12b' of the common electrode 12' by a certain distance.
Finally, an overcoat layer, and a protective layer for covering the overcoat layer are formed (See Fig. 2). The overcoat layer is formed by a thick film technique. The protective layer is so formed as to extend not only onto the upper surface but also continuously onto the bevel surface 10c' and the side surface 10b' of the substrate 10'. The protective layer may be formed of sialon for example (or a material containing sialon) in the form of a thin film.

Claims

A thermal printhead comprising:
an insulating substrate (10) having an upper surface (10a) and a side surface (10b, 10b');

a heat retaining glaze layer (11, 11') formed on the upper surface (10a) of the substrate (10, 10');

a heating resistor (13) formed on the glaze layer (11, 11');

a common electrode (12, 12') including a plurality of teeth (12a) connected to the heating resistor, and a connecting portion (12b, 12b') connecting the teeth (12a) with each other;

a plurality of individual electrodes (15) connected to the heating resistor (13);

an electrode auxiliary layer (14, 14') formed on the connecting portion (12b, 12b') of the common electrode (12, 12');

an overcoat layer (16) for covering the heating resistor (13) and the electrode auxiliary layer; characterized in that the thermal printhead further comprises;
a protective layer (17) for covering the overcoat layer (16); and
that the connecting portion (12b, 12b') of the common electrode (12, 12') includes a first region contacting the glaze layer and a second region contacting the upper surface (10a) of the substrate (10, 10').
The thermal printhead according to claim 1, wherein the electrode auxiliary layer (14, 14') contacts both the first region and the second region of the connecting portion (12b, 12b').
The thermal printhead according to claim 2, wherein the electrode auxiliary layer (14, 14') includes a thinner portion contacting the first region of the connecting portion (12b, 12b') and a thicker portion contacting the second region of the connecting portion (12b, 12b').
The thermal printhead according to claim 3, wherein the protective layer (17) includes a first protrusion (17a) positionally corresponding to the heating resistor (13), and a second protrusion (17b) positionally corresponding to the thinner portion of the electrode auxiliary layer (14), the first and the second protrusions (17a, 17b) being substantially equal in height.
The thermal printhead according to any one of claims 1 to 4, wherein the glaze layer (11, 11') includes an uneven portion (11c) contacting the first region of the connecting portion (12b, 12b'), the uneven portion (11c) being tapered toward the side surface (10b, 10b') of the substrate (10, 10').
The thermal printhead according to any one of claims 1 to 5, wherein the substrate (10, 10') has a bevel surface (10c, 10c') extending between the upper surface (10a) and the side surface (10b, 10b') of the substrate (10, 10').
The thermal printhead according to claim 6, wherein the glaze layer (11, 11') is spaced from the bevel surface (10c, 10c').
The thermal printhead according to claim 6 or 7, wherein the bevel surface (10c, 10c') is covered with the protective layer (17).
The thermal printhead according to any one of claims 6 to 8, wherein the bevel surface (10c, 10c') is roughened.
A method of making the thermal printhead according to claim 1, the method comprising the steps of;
forming the glaze layer (11, 11') to be spaced from the side surface (10b, 10b') of the substrate;
forming the common electrode (12, 12') and the individual electrodes (15) on the glaze layer;
forming the electrode auxiliary layer (14, 14') on the common electrode (12, 12');
forming the heating resistor (13) in contact with the common electrode (12, 12') and the individual electrodes (15);
forming the overcoat layer (16) to cover the heating resistor (13) and electrode auxiliary layer (14, 14'),
characterised in that;
the overcoat layer (16) is spaced from the side surface (10b, 10b') of the substrate;
the connecting portion (12b, 12b') of the common electrode is formed to include a first region contacting the glaze layer (11, 11') and a second region contacting the upper surface (10a) of the substrate (10, 10') ; and
the method further comprising the step of forming the protective layer (17) to cover the overcoat layer and the second surface of the substrate.
The method according to claim 10, wherein the glaze layer (11, 11') is formed to have an uneven portion (11c) tapered toward the side surface (10b, 10b') of the substrate (10, 10'), the first region of the connecting portion (12b, 12b') of the common electrode (12, 12') being formed to contact the uneven portion (11c).
The method according to claim 10 or 11, wherein the step of forming the electrode auxiliary layer (14, 14') includes applying a fluid conductive paste onto both the first and the second regions of the connecting portion (12b, 12b') of the common electrode (12, 12').
The method according to claim 12, wherein the conductive paste is allowed to flow from the first region to the second region.
The method according to any one of claims 10 to 13, wherein the substrate (10) has a bevel surface (10c) extending between the upper surface (10a) and the side surface (10b) of the substrate (10).
The method according to any one of claims 10 to 14, further comprising the step of working the substrate (10') to provide a bevel surface (10c').
The method according to any one of claims 10 to 15, wherein the insulating substrate is obtained by dividing an insulating support (20'), the method further comprising the steps of:
cutting the support (20') at a position spaced from the common electrode (12') and the electrode auxiliary layer (14') after the step of forming the electrode auxiliary layer;

chamfering the support (20') to have a bevel surface (10c') which is spaced from the common electrode (12') and the electrode auxiliary layer (14').
The method according to claim 16, further comprising the step of applying laser to the support (20') from below to form a cutting guide groove (CL) for cutting the support (20').
The method according to claim 16 or 17, wherein the protective film (17) is formed of a material containing sialon.